by Dr. Randall K. Kirschman, consulting physicist, Silicon Valley, extelect@gmail.com

This is Part 2 of a two-part article about cryogenic semiconductor electronics. Part 1 appeared in the preceding Cold Facts (Vol 37, No 1); it gave a brief introduction and presented applications of cryogenic electronics for sensors and spacecraft, microwave receivers, and power conversion. This part presents additional application examples, difficulties, and some closing comments.

Faster computing
Decades ago, cryogenics was considered for computers because cooling of semiconductor logic circuits was found to increase their speed. A typical increase was a factor of ≈ 2 upon cooling to approximately LN2 temperature. Circa 1986 several “supercomputers” were built to take advantage of this factor of ≈ 2 (Figure 9).[17] Circuits optimized for cryogenic temperatures might have achieved a speed advantage of three or more, but this was never implemented on a practical scale.

Moreover, computers evolved in other directions that left cryogenic operation as another forgotten venture. Recently, however, cryogenic cooling has re-emerged in conjunction with both semiconductor-based and superconductor-based computing, inspiring interest in cryogenic memory systems based on semiconductor devices (as well as other technologies) for increased access speed and reduced power dissipation.[18] Also, a new and active application area has emerged: quantum computing.

Quantum computing
Cryogenic electronics for computing is taking a “quantum leap” to an area that is expanding rapidly in importance: to support quantum bits (qbits). To protect their fragile quantum states, qbits must be isolated from sources of thermal noise. For qbits based on superconductors or semiconductors this involves cooling to mK temperatures. Directly interfacing such qbits to room-temperature electronics risks disturbing the quantum states via thermal noise, which must be mitigated by extensive filtering and progressive cooling of the incoming lines to mK temperatures.

The hazardous journey of weak qubit signals to room temperature is similar to that described in Part 1 of this article for sensors. Ensuring signal integrity between the qbits and room-temperature is vital, as well as reducing interconnection complexity, and minimizing the associated unwanted heat transfer. These issues would become unmanageable as the number of qbits is scaled up from the current dozens to millions. Thus, it is no surprise that co-location is being implemented for semiconductor electronics near the quantum circuits for communication, control, and interfacing with room-temperature electronics (Figure 10).[19] Operating the semiconductor electronics near the quantum electronics and at similar very low temperatures requires extreme ingenuity to avoid a large heat load on the mK refrigeration and to avoid disturbing the qbits.

Space exploration
Very little of the Solar System—or the Universe—is at room temperature. Many sites of interest for exploration would subject landers and rovers to cryogenic temperatures (Figure 11). E.g. a Mars rover could experience temperatures as low as ≈ 140 K, and even our nearby Moon registers ≈ 50 K in the Aitken Basin at its south pole.

In such cryogenic environments, it would be advantageous to operate part, or all, of the electronics of landers and rovers at the environmental temperature. This would allow reduction or elimination of the heating system and its power burden needed to maintain the electronics at “normal” temperatures. Added benefits are higher reliability, lower launch mass, and less waste heat that could disrupt the very environment being explored. In addition, mission lifetime might be extended, or missions that would otherwise be impractical might be made possible.

Cryogenic electronics has seen wide-ranging use in scientific, free-flying or orbiting spacecraft, as described in Part 1; however, its use in landers and rovers has so far been very limited.

Biological cryo-preservation
In a different realm, an example of semiconductor electronics for an existing cryogenic environment (in this instance artificial) relates to tracking and handling of biological specimens. In a system developed by Fraunhofer IBMT (Institut Biomedizinische Technik), cryostats accommodated on the order of 10,000 vials for cell-specimen storage, cooled to ≈ 90 to 125 K by liquid nitrogen vapor. Each vial (Figure 12, left) incorporated a 256 k memory integrated-circuit “chip.”[20] This arrangement could be considered co-location, although not for an electrical connection, but for a physical association.

To electronically identify and access the memory chips—and thereby track the temperature and history of the specimens—a sophisticated electronics system, capable of reading and writing to each memory chip while cold, was resident in the cryostat (Figure 12, right). The electronics included microprocessors, random access memory, programmable logic, opamps, analog switches, and power management. The electronic components operated at a similar cryogenic temperature to that of the vials, but were tested to operate down to 77 K. This was probably the most complex semiconductor electronics system operated at cryogenic temperatures.

Difficulties
Cryogenic electronics has found many practical applications, but there can be many difficulties to deal with in operating electronics at reduced temperatures. Although thermal energy is generally disruptive, it is sometimes beneficial or enabling.

First, in certain situations, lack of thermal energy may inhibit movement of charge carriers: (a) Semiconductors may become non-conductive at very low temperatures. For example, Si can become an insulator below ≈ 40 K; this condition is referred to as “freeze-out.” Appropriate designs and use of materials are needed to overcome freeze-out and enable transistor operation down to the lowest temperatures; (b) At low temperatures, charge carriers may become “trapped” in energy wells caused by imperfections or impurities within a transistor’s structure or within a dielectric material. The trapping may be cumulative and effectively permanent in practice, resulting in parameter variation over time, or the trapping may be short-term, resulting in random charge movement that manifests itself as extra noise or other undesirable effects.

Passive electronic components are not immune to problems: e.g. capacitors may suffer reduced effectiveness—as much as an order-of-magnitude lower dielectric constant—because some charge-storage mechanisms depend upon atomic or molecular movement aided by thermal energy, and these may “freeze out” (Figure 13).
(This is different from semiconductor “freeze-out” which involves charge-carrier flow.)

Batteries also suffer, since decreased thermal energy diminishes electrochemical processes. The lower limit of chemical battery operation is about 200 K, although R&D may push this down to ≈ 130 K. Even so, performance is severely degraded.[21] Lack of an independent power source that can operate at cryogenic temperatures is an impediment to several possible applications of cryogenic electronics; e.g. self-contained, autonomous observation units distributed on a cold planet.

Finally, there is the challenge of interfacing between the low-temperature and room-temperature electronics, e.g. as mentioned in connection with quantum computing. This involves power transfer, signal transfer, and heat load on the cryogenic cooling system.

Why isn’t cryogenic electronics more widely used? In many applications electronics could deliver better performance via cryogenic operation. In my view, a major deterrent is providing the cryogenic environment. Cryogenic electronics might enjoy wider use if small (mW), reliable, self-contained refrigerators for the range 20-120 K, were available. Why not use thermoelectric coolers? These are widely used for moderate cooling, but at present are unable to cool to cryogenic temperatures. The largest ΔT available is about 60 °C for a single-stage unit and about 120 °C for a multistage unit. Considering that the lifted heat (plus the heat powering the cooler) must be dumped at a temperature above ambient, the lowest presently attainable temperature might be only as low as ≈ 200 K.

Looking ahead
Cryogenic semiconductor electronics spans a large frequency and power range in a wide variety of uses, present and future: from nearly DC to mm-waves; from microwatts to megawatts. It can range from a single-transistor circuit to a system with many complex circuits. Cryogenic electronics is not a “laboratory curiosity,” but has a variety of widespread applications “in the field.” This article has presented a few examples from among many, and there is much more to cryogenic electronics.

Electronics will continue to progress and advance into new areas, but thermal energy is always a factor—often disruptive—and lowering operating temperature will remain an option for enhanced system performance. Wherever minute energies are involved and thermal disturbances need to be suppressed, or wherever parasitic losses need to be reduced in power systems, operating electronics at cryogenic temperatures will continue to be an option when enhanced performance is required and when the attendant difficulties and expense can be justified.

Additional information on cryogenic electronics can be found on (a) the CSA website in the Cryo Central section under Cryogenic Electronics, and on (b) the author’s website (see biographical paragraph at end) including meetings, notices, and sources.

Acknowledgements
I am indebted to Frank Ihmig (Fraunhofer IBMT) for reviewing this article and for providing valuable suggestions and photos, to Murzy Jhabvala (NASA Goddard Space Flight Center) for reviewing this article and for providing valuable suggestions, and to Teledyne Imaging Sensors for providing an image of the SIDECARTM ASIC (Figure 3 in Part 1).

References
[17] D. M. Carlson, D. C. Sullivan, R. E. Bach and D. R. Resnick, “The ETA10 liquid-nitrogen-cooled supercomputer system,” IEEE Trans. Electron Devices, v. 36, n. 8, pp. 1404-1413, Aug. 1989; doi: 10.1109/16.30952
[18] D. S. Holmes and S. W. Van Sciver, “Refrigeration systems for quantum, cryogenic computing,” Cold Facts, v. 36, n. 6, pp. 8-10, Dec. 2020.
[19] S. J. Pauka, K. Das, R. Kalra, A. Moini, Y. Yang, M. Trainer, A. Bousquet, C. Cantaloube, N. Dick, G. C. Gardner, M. J. Manfra and D. J. Reilly, “A cryogenic interface for controlling many qubits,” arXiv:1912.01299v1 [quant-ph] 3 Dec. 2019; http://arxiv.org/pdf/1912.01299.pdf
[20] F. R. Ihmig, S. G. Shirley, R. K. Kirschman and H. Zimmermann, “Frozen cells and bits – Cryoelectronics advances biopreservation,” IEEE Pulse, v. 4, n. 5, pp. 35-43, Sept./Oct. 2013; doi: 10.1109/MPUL.2013.2271685
[21] M. C. Smart, B. V. Ratnakumar, W. C. West and
E. J. Brandon, “Primary and secondary lithium batteries capable of operating at low temperatures for planetary exploration,” Lunar Superconductor Applications – 1st International Workshop, Houston, Texas, 3-5 March 2011; http://trs.jpl.nasa.gov/bitstream/handle/2014/41983/11 0823.pdf?sequence=1
[22] F. Teyssandier and D. Prêle, “Commercially available capacitors at cryogenic temperatures,” Proc. – Ninth International Workshop on Low Temperature Electronics (WOLTE 9), Guarujá, Brazil, 21-23 June 2011, pp. 93-96, HAL archives-ouvertes 00623399; http://hal.archives-ouvertes.fr/hal-00623399/document

Dr. Randall Kirschman is based in Silicon Valley and provides consulting related to R&D for electronic devices and circuits, as well as assistance in obtaining funding, particularly for extreme temperatures. He also presents professional development courses covering the principles and practice of low-temperature and high-temperature electronics. He received his Ph.D. from Caltech in physics and electrical engineering. E-mail: ExtElect@gmail.com, Website:
http://www.ExtremeTemperatureElectronics.com

Editor’s Note: The y-axis labels of Figures 8a and 8b in Cryogenic Electronics: Part 1, were unidentified. For each graph the left y-axis label should be “Thermal conductivity (W/cm-K)” and the right y-axis label should be “Thermal conductivity (W/m-K).” ■